Current-controlling and static-reducing system



Dec. 11, 1923.

C. A. SPRAGUE CURRENT CONTROLLING AND STATIC REDUCING SYSTEM JPACE CURRENT Filed May 2 1919 4 Sheets-Sheet. 1

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C. A. SPRAGUE CURRENT CONTROLLING AND STATIC REDUCING SYSTEM Filed May 2, 1919 4 Sheets-Sheet 5 have H fo r: C/are nae 4. 6pm; ue

C. A. SPRAGUE CURRENT CONTROLLING AND STATIC REDUCING SYSTEM Dec. 11 1923. 1,477,017

Filed May 2, 1919 4 Sheets-Sheet 4 //7 vemor: C/arence ,4. 5pm as by W Patented lDec. M, 1923.

NrrEo STATES CLARENCE A. SPRAGUE, OF EAST ORANGE, NEW JERSEY, ASSIGNOE T0 WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION OF NEW YORK.

CURRENT-CONTROLLI NG AND STATIC-REDUCING SYSTEM.

Application filed MayZ, 1919. Serial No. 294,328.

To all whom it may concern. Be it known that I, CLARENCE A. Srnxouc. a citizen of the .United States, residing at East Grange, in the county of Essex, State of New Jersey, have invented certain new and useful Improvements in Current-Controlling and Static-Reducing Systems, of which the following is a full, clear, concise, and exact description.

The present invention in its broad aspect relates to a methodand means for combining the properties of a pluralty' of electric discharge devices, or their equivalents, arranged in parallel relation, to secure a desired resultant property. It involves con-- trolling the order and extent of operation of the discharge devices, which may, for example, be thermionic vacuum tube devices.

In a more specific aspect the invention relates to the reception of relatively weak signal energy in the presence of large electrical disturbances. It is particularly adapted to the reception of electromagnetic signal waves at radio receiving stations during atmospheric or other disturbances, but is not limited to such use, since it is applicable also, for example, to wire'or cable signaling systems in which disturbing current waves are received together with the signal waves.

The invention is designed particularly for overcoming the disturbing effects of abnormal surges of energy of low periodicity compared with the signal currents. These relatively slow impulses of great amplitude are very troublesome and are difficult to control not only in radio signaling, but in high. frequency carrier current wire systems and in systems of great capacity, such as the submarine cable. Without some correcting means for these large undesired currents, the receiving instrument or the operators car may be temporarily rendered insensi-' tive or permanently injured, or at least it may be impossible to receive the signal during the persistence of the disturbances owing to its being swamped by the disturbingwaves. Protective devices are known for preventing toolarge current from reaching the receiver but such devices wipe out the message as well as the excess current. It is an object of the invention to prevent too large current from reaching the receiver, but at the same time to permit. the signal currents to be received.

A feature of the invention relates to the method of and means for attenuating currents of abnormal magnitude while permitting signal currents to be received.

Another feature of the invention relates to the control of a signal current by the abnormal current for determining one or another path for the passage of the signal currents.

A further feature of the invention relates to the separate treatment of the normal sig nal currents and the currents received during the disturbances.

Features concerned in securing the abovementioned effects relate to the use of discharge devices so arranged as to function sequentially or in steps in accordance with the magnitude of the received energy, to modulate signal waves in accordance with abnormal currents, and to control attenuation of the abnormal currents; andother features of the invention will appear from the following description together with the drawings, of which Fig. 1 shows a receiving circuit arrangement according to the invention; Figs. 2 and 3 are curves explanatory of the action of the device of Fig. 1; Fig. 4

is a modified circuit arrangement of which the curves in Fig. 5 are explanatory; Figs. 6 and 7 are other forms of circuit arrangements which the invention may take, and Fig. 8 illustrates a high power modulator or detector of which the curves of Fig. 9 are explanatory.

Referring to Fig. 1 the antenna 1 is connected through adjustable resistance 2 to earth and is coupled by coil 3to the receiving circuits shown. Resistance 2 is preferably adjusted to a value which will render the antenna circuit non-oscillatory. 4:, 5 and 6 indicate electron discharge devices and as illustrated comprise evacuated tubes containing heated filaments 7 and anodes or plates '8. The filaments-are supplied with heating current from battery 9 connected through regulating resistances as shown, or by separate batteries if desired. Plate battery 10 causes a space current to flow norapproximately that indicated by the abscissa a so that the normal spacecurrent would be represented by the length of the ordinate drawn at a. If signal electromotive forces impressed through coupling 3 and superimposed upon that of battery 10 should-cause the total space potential to vary between the limits indicated by b,.0, the variations produced in the space current by the signals would be represented by the difference between the ordinates drawn at points I) and c. If a surge of current from some disturbing source should carry. the space potential to some point (i and maintain it'there momentarily, the signal variations coming in with the disturbing current would produce no variation in space current, since the saturation point of the characteristic curve A of the discharge device would have been passed, and hence the signals would be lost during the existence of the disturbing current. Discharge device 4 will act then as a current limiting device, since fora given filament temperature no larger current can be made to flow through it than is represented by'the ordinate corresponding to the horizontal portion of curve A.

Electron discharge devices 5 and 6 are substantially like discharge device 4 but are connected to the common space .battery 10 through respective resistances 12, 13 and potential sources 14 and 15. As shown, source 14 is of less potential than 15 and both sources are connected so as to oppose the source 10,; so that the normal spacetpotential of discharge device 5 is less than that O of discharge device 4 and that of discharge device 6 is less than either of the others. Also the amount of resistance 13 included in the circuit may be assumed to be greater than the portion of resistance 12 that is used. The action of discharge devices 5 and 6 is to permit currentto flow in the common output circuit after the cut-off value of the discharge device i ha been reached; With a space potential d for ihstance, a larger current results than is denoted by the curve A as indicated by the dotted curve B and the ordinate drawn at d. However, the current fiowing through discharge device 5 or discharge devices 5 and 6 is attenuated by the resistances 12, or 12 and 13, and the attenuation is greater the greater the currents, as is indicated by the general form of the curve B which may be considered the composite characteristic of the group of parallel discharge devices 4, 5 and 6 as a whole. 'A-large surge of disturbing current is thereforegreatly damped or reduced but the signals are not lost, since it may be assumed that saturation is not reached. The effect is'graphically shown by the curves of Fi #3, in which the small irregular curve in icates the received si nal current. To the left of the line D conditions are normal and'only a signal wave is present. To the right of the line D, however,a slowly rising irregular wave of great magnitude is repre-' sented as coming in together with the signal, the result being the large wave E with an irregular outline due to the superposition of "current, however, and all such excess current is attenuated by the resistance 12, or, if both discharge devices 5 and 6 are conducting current, by the resistances 12 and 13, and

the result 1s that the disturbing current is reduced to some such extent as is indicated by the curve E which it is assumed represents the wave actually transmitted to the output coil 11.

It is evident that the signal wave will also be attenuated but as long as the curve B continues to possess curvature, the signal currents will still be present and are transmitted through windings 11, and 16 to the input circuit of the detector 17 of familiar type and are rendered audible in thereceiver 18. A tuning condenser 19 may be used or the detector circuit may be made aperiodic or detuned by opening the key 20. By thus greatly attenuating the disturbing current this circuit arrangement prevents a sudden large fluctuation of current from reaching the ear of the operator. Owing to the curvature of characteristic B, Fig. 2, the devices 4, 5 and 6 .will act in a measure as a detector. The low frequency signal currents thus set up in the input circuit of detector 17 will in part reappear in the telephone receiver 18, by virtue of the repeating action of detector 17, and will, in part, act

to modulate the carrier current component, the resultant current not being effective in reproducing the signal in the receiver 18. Circuit modifications in which this loss of energy does not occur will later be described.

The relative effects upon the operators ear of the currents produced by static disturbances and those due to signals may be likened to the effect upon the eye of a powerful field of light superimposed upon which is a small weak source of light. strong field is very intense, the eve will be blinded and become incapable of discriminating between the different intensities.

If the If, however, the intensitiesof both fields are reduced in the same proportion, there is a ing to the'form of this invention just described, the intensity of the signal and the static are reduced in about the same proportion, with a resulting increase in the clearness of the signal. According to other forms of the invention the static energy is very greatly reduced while the available signal energy is reduced a relatively small amount or is increased.

The system of Fig. 1 which has been described in detail with reference to a disturbing wave of such character as to raise the space potential above that of suI'ce would need some modification to m'akeit equally efficacious in reducing disturbances a of reverse "sign. Circuit arrangements will now be described which are designed to re duce a disturbing current of either polarity and in which the detecting action of the current limiting device is taken into account. Referring to Fig. 4, in place of the twoelement discharge devices 4, 5 and 6, discharge devices 21, 22 and 23 of the ther-' mionic relay type are shown, each containing in addition to the hot filament and the anode, a third or grid electrode 24, 25 or 26 respectively. The grid 24 is provided with a polarizing potential source 27 to fix its normal potential with respect to its filament 7 and its potential is indicated as higher than that of its filament. Similarly, by a source 28, the normal potential of grid 26 is fixed negative with respect to its filament 7,

while the potential of grid 25 is normally the same as that of its filament 7 Source 10 provides a normal potential difference across the space between filament 7 and anode 8 of each of the discharge devices 21, 22 and 23. If the;

characteristic curves of the discharge devices 21, 22 and 23 are sis-represented by the respective curves F, G and H, in Fig. 5, "'then a normal space current will flow through each discharge device n response to this normal space po-tential,.the respective magnitudes of the space currents through the several discharge devices being indicated by the length of the ordinate intercepted on the line 0 by the respective curves F, G and A summation curve K obtained by adding curves F G and H, may serve as the characteristic curve of the three discharge devices 21, 22 and '23 in combination as shown. The ,zero' given for these curves indicates the points on their characteristics corresponding to their respective fixedpotentials and the abscissae indicatevariations of grid potential from their respective fixed grid potentials. Incoming signal waves vary the potentials of all three grids between the limits it and k, for instance. Those signal variations produoe'very slight changes in the plate currents through discharge devices 21 and 23 as shown by the relatively fiat ortions of their characteristics F and H respectively, that are operated over between these limits, as determined by the potential sources 27 and 28 respectively. However, the variations of plate current through discharge device 22, due to changes in'its gridpotential between these limits b, and k are considerable, as shown by the curve G, and the resultant curve K shows that in the comparatively linear region of the curve K between limits it and 7c the variations in the space current are very nearly proportional to variations in grid potential and the discharge device 22 is acting as an amplifier. These variations of plate potentialas determined by the signal waves are transmitted through the coupled windings 29 into the input circuit of a detector 30 of familiar type and are rendered audible in the receiver 31. Thus the normal signal waves picked up by antenna 1 are amplified by discharge device 22 in the ordinary manner, detected by detector 30 of some suitable form and received at 31 practically without any effect due to discharge devices 21 and 23. However, if a large disturbing wave carries the grid potential variation to such excessive values as either that indicated at m on the one hand or n on the other hand, one of the discharge devices 2L or 23 comes into. play. It will be noted that the characteristics F and H of discharge devices 21 and 23 respectively, are not so steep as the curve G and that the saturation region for these ,two discharge devices is reached relatively more suddenly than in the caseof discharge device 22. This may be due to the effect 'of the large external resistances .32 and 33 and not to inherent difi'erences in the devices themselves. The large disturbing wave. assumed to-have "aried the grid potential to the extent indicated first by m brings the discharge device 22 up to the point of its ing devices permitting only current avithin predetermined limits to pass. while discharge device 23 detects the signal variations and the variable currents which it transmits are attenuated by the resistance 33 before they reach receiver 34. A disturbance in the opposite direction to'the extent indicated by the principal action is that of ziliipliiication.

indicated by the substantially linear portion of the characteristic between these limits, but that beyond these limits in either direction the curve begins to fall oil and the .total current transmitted bears an increasingly smaller ratio to the incoming currents, but detection of the signals still continues by virtue of the curvature still possessed by the characteristic. lVhen working over certain small ranges of curve K. signalsniay be audible in'both receivers. The phase shifting means 43 may then be adjusted to obtain the proper phase relations.

A generator of carrier or other high frequency may, if desired. be coupled to the circuit through coil- 36'shown inductively related to windings for the purpose of increasing the strength of the signal or otherwise rendering it more easily readable. A key 37 is provided in the circuit of the generator 35. Alsoa tuning condenser 38 maybe th'rown into or out of circuit across .the secondary of windings 3 by key 39. It

may be found desirable to .render this tuned circuit non-oscillatory by the insertion of suitable resistance. A tuning condenser 40 is shown in circuit with the secondary of transformer 29. The small condensers 41 and 42 are shown respectively in shunt to receivers 31 and 34 to by-pass the high frequency current components. The direct currentfrom battery 10 may in any case be shunted past the indicating instrument by choke coils in the well-known manner.

The devices thus far described are de-' signed to reduce the effect. of disturbing currents, while still permitting the signal currents to be received but the signal currents are somewhat attenuated by the current reducing means. In the remaining figures means are shown for reducing the disturbing currents while still transmitting the signal currents strongly. This is accomplished' in the particular means shown by causing .the lower frequency disturbing currents of large magnitude to control amplification of the signal currents, after which both kinds of currents are attenuated. Referring first to Fig. 6. the secondary of the antenna coupling 3 is connected to the input" of a balanced. amplifier commonly known as the push-pull type. This con:- prises two discharge devices 45 and 46, the respective grids of which are connected each to a terminal of the secondary of coil 3, while their respective filaments 49 and 50 are connected together and to the midpoint of the said secondary winding. A given induced potential in said secondary will therefore raise the potential of one gridrelative to its filament and will lower the potential of the other grid by an equal amount. The respective anodes or plates 51 and 52 are connected to the extremities of the primary winding of output coil 53 and source ltl of space current is connected to the common lilament connection and through a con'ibination of discharge devices to the midpoint of said primary winding. The combination of discharge devices in the space current circuit as shown is the same as that of Fig. l and the identity of the various parts may be recognized by the same reference characters as are used in Fig. 1. The action of this combination of discharge devices as described in connection with Figs. 1 and 2 is to limit the current in their common circuit. not by cutting it off definitely at a particular saturation value but at a varying rate somewhat as indicated by the dotted curve B of Fig. 2, so that the signal current variations upon the disturbing waves are not lost. The normal potential of grids 47 and 48 is fixed preferably negative by the source of potential 54 so that either device 45 or 46 normally operates over that portion of its characteristic near the transition from the lower horizontal portion to the substantially linear upwardly sloping portion. An incoming signal wave will render one grid, say 47, at any given instant. more negative than normally and since the impedance of this discharge space is already practically a maximum, due to its large normal negative potential, there will be substantially no change in its output current. The same wave will at the same instant make the potential. of grid 48 less negative and therefore the impedance of its discharge space becomes lower, and an output wave, depend ing upon the extent to which the grid potential is changed, will result. For an incoming wave of alternating current then the discharge devices 45 and 4G alternately conduct current under control of the input waves. The transmission of space current under control of signal current, while it may be regulated to be normally adequate for the reception of signals, is less favorable than if either discharge device 45 or 46 were operating over a more nearly linear and central portion of its characteristic.

This more favorable condition is produced in one or the other discharge device 45 or 46 by a sufiiciently large disturbing wave of relatively low frequency. since the potential of one or the other grid will be raised momentarily. The corresponding lowering of the potential of the other grid will cause no further increase in the already practically maximum impedance of its discharge space,

are modulated in accordance with the lower that the possibility duce no variations in frequency disturbing waves, and the signals become stronger the stronger is the disturbing current. As in the case of the arrange-- ment of Fig. 1 the discharge device 4 transmits the normal signal currents, but larger currents than normal begin to find their way through the discharge devices 5 and 6 and become attenuated by the resistances 12 and 13 so that all variable currents of abnormally large amplitude in the common output of the discharge devices and 46 are reduced before they reach the output windings 53 through which they are impressed upon the detector 55 of familiar type bywhich they are rendered audible in the receivers 56. The signal currents by having been previously boosted may still be as strong as normal, although they have been attenuated together with the disturbing currents.

It will be noted that in the arrangements shown, sharply tuned oscillatory circuits are avoided throughout the path of the received waves until after the waves have passed the current limiting and attenuating means, so of shock-excitation of a resonating circuit .by a sudden disturbance, whereby an aperiodic impulse is converted in the resonating circuit, is avoided. After the oiirrents have been attenuated there is less probability of this effect and a tuning condenser in the detector circuit may be advantageous in selectively transmitting only the signal cur rents. Condensers 19, 40 and 57 are indicated to be used for this purpose if desired, Any suitable form of wave filter may, of course, be used in place of a single tuned circuit.

Referring now to Fig. 7 the antenna coupling 3 is connected to the input circuit of a detertor 58 of familiar type by which the signals are rendered audible in receiver 59. A tuning condenser 60 may be included around the coil winding by closing key 61 if desired. A coupling 3 is also shown, the secondary ofwhich is connected to the input terminals of an amplifier of the push-pull type similar in general to that of Fig. 6 except that. the normal grid potential is fixed at such a large negative value by bat tery 62 that the normal signal currents prothe common output coil 63. The normal currents are transmit ted by the detector 58 and the disturbing currents, if of such large value as to exceed the cutrofivalue of the detector 58 which value may be suitably predetermined, will change the potential of the grids 64 and 65 of the push-pull amplifier sufiiciently to enable one or the other side of said amplifier to transmit the signal-current. The larger the distributing current within reasonable limits, the greater will be the amplification of the signal currents as explained in connection with Fig. 6 and the signal currents are thus so augmented that the signals are still strong after the currents have passed the large attenuating resistance 66 in the output of the amplifier. By means of windings 63 and 67 the output waves are impressed on the input circuit of detector 68 by which they are rendered audible in receiver 59. A tuned condenser 69 is shown which may be connected across the coil 67 by closing key 70 if desired.

Fig. 8 discloses a modulator similar to that of Fig. 7 with the addition of tubes 81 to having the required grid potentials and output resistances to give the operating characteristics shown in Fig. 9, the characteristics of tubes 65, 80, 82 and 84 being respeotively the same as those of tubes 64,81, 83 and 85. The source of electromotive force 62 is of such value, as in Fig. 7, as to normally prevent flow of space current. The two groups of tubes therefore act alternately as the impressed eleotromotive force reverses in direction. The operating characteristics of the tubes of either group beginning with 64 or 65 are shown at R, S, '1 and V respectively, Fig. 9, and W is the characteristic of the combination It will be noted that the curve lV may be made substantially parabolic over a. relatively very wide opera-ting range and that this modulating system is therefore well adapted for powerful currents. The system may also be used as a detector, and, with proper feedback circuits, will function as a power oscillabor. The necessary coupling may be obtained by inductively relating coils 63 to coils 86, for example.

In some. of the modifications described above two telephone receivers are disclosed. Ordinarily these may be combined in a, single instrument, as in Fig.

of indicating or recording instrument may be employed. The inventions herein disclosed are well adapted to photographic or tape recording systems or other systems in which a visual record is obtained, as the amplitude of the static or other interfering, waves is reduced to a point where a record can alwavs be obtained without obliterating the signal.

While the invention has been illustrated and described with particular reference to a receiving circuit for electromagnetic waves, it is not to be limited to such a systern since it is evident that the antenna. 1 may be equally well considered as the terminal of a wire line. Also the particular discharge devices illustrated are not to be t, if desired. in place of telephone receivers, any other type I i the relations set out 1n the various curves age space-current that have been given to be considered as limiting the invention but merely as illustrative of principles involved and as aids to a disclosure of the invention. Nor isthe inventor to be limited in other respects by the specific embodiments shown as a whole' or to the particular details, but only by the scop e of the appended claims.

hat is claimed is:

1. The method of reducing disturbing current which comprises transmitting portions of said current determined by the itude of said current, and attenuating sai current portions to different degrees,

2. The method of reducing disturbing current which comprises separately and simultaneously attenuating different portions of said current in accordance with its magnitude.

3. The method of signaling in the presence of abnormal current which comprises normally confiningthe'signaling currents to a low impedance path but causing both signaling currents and abnormal currents when both are present to follow a higher impedance pat i Y 1. The method of reducing the effect of abnormal current upon the reception of signal currents which comprises separately relaying portions of said signal currents in accordance with the strength of the abnormal current.

5. The method of receiving signals in the presence of .disturbing impulses comprising applying received signal and disturbing energy to a plurality of detector circuits in parallel, operating said detectors so that the maximum response of respective detectors occurs at respectively larger amplitudes of applied energy, attenuating to a reater extent the detected current from t e detectors which ivegreater response tothe larger amplitude impressed energy, and applying the detected cur.- rent tromall of said detectors in common to an indicating device.

6. The method of combining the properties of a lurality of discharge devices to secure a esired resultant property which comprises connecting said discharge devices in parallel relation and causing at least a portion of them to operate in sequence, and, in part at least, with different input volt- .characteristics.

controlling circuit. a discharge devices having difier- 7. In a .current plurality of ent input-voltage space-current charactistics and connected in parallel relation and provided with controlling means for causing said devices to operate sequentiall in accordance with the magnitude of sai current.

8. In a variable current circuit, a plurality of discharge devices operable in sequence to transmit said variable current, and means external to said devices for making certain of said devices respond to impressed potentials of only one polarity, and other of said devices to potentials of only opposite polarity.

9. In a system for controlling transmission of current variations, a plurality of discharge devices having a common input connection and a common output connection, and means for attenuating the current variations in varying amounts, connected in circuit between said common output connection and the several said devices.

10. In a circuit for controlling transmission of current variations, a path for the current variations comprising a plurality of parallel branches each said branch comprising a discharge space and a resistance in series, said resistances having different values.

11-. In a circuit for controlling transmission of current variations, a plurality of dischar e devices each having a cathode, an ano e and an impedance controlling element, a common input circuit connected to said cathodes and said impedance controlling elements, a common output circuit for the current variations connected to said cathodes and said anodes, said several impedance controlling elements being normally at different potentials with respect to their several cathodes, and means connected between the anodes and the cathodes of said devices for attenuating to a greater extent the current variations transmitted by certain of said devices than the current variations transmitted by certain other of said devices.

12. Signal receiving means comprising signal-current-'ntercepting means, a receiver for said signals, and attenuating means between said intercepting meansvand said receiver. actuated selectively in steps in accordance with the magnitude of'the intercepted current.

' 13. Signal receiving means comprising current-intercepting means for both signal currents and other currents, means for re ceiving said signal currents, and means for suppressing said other currents comprising a plurality of current-limiting and current attenuating paths in parallel.

14.. In a signal receiving circuit, a receiving terminal, receiving means-and a plurality of paralled transmission paths between said terminal and said receiving means, one said path being of small attenuation and containing an amplifier and a detector and another said path being of relatively high attenuation and containing a detector.

15. In a signal receivin circuit traversed by signal currents and 0t 'er currents, a receiver responsive to any substantial flow of signal current therethrough, a discharge device normally opaque to said signal currents but arranged to conduct said signal currents to said receiver when said other currents are large compared with said signal currents.

16. In a signal receiving system, means for suppressing disturbing currents comprising a current limiting device for transmitting currents only below a predetermined amplitude, and a current-limiting and attenuating path in parallel thereto for attenuating currents above said predetermined amplitude.

17. In a signal receiving circuit, an incoming terminal, a receiver responsive to any substantial flow of signal current therethrough, a path for signal currents between said terminal and said receiver comprising a plurality of discharge devices in parallel relation and means to cause said discharge devices to operate in sequence to transmit said signal currents to said receiver, in accordance with .the magnitude of the received current.

18. In a signal receiving system, an inooming terminal, signal receiving means re-v sponsive to any substantial flow of signal current therethrough, said terminal being traversed by both signal currents and other currents, a plurality of paths for saidsignal currents between said terminal and said receiving means to transmit signal currents to said receiving means, and means actin in accordance with the strength of said other currents to determine the path or paths taken by said signal currents.

19. In a signal receiving system, an incoming terminal, signal receiving means responsive to any substantial flow of signal current therethrough, said terminal being traversed by both signal currents and other currents, a plurality of paths for said signal currents between said terminal and said receiving means to transmit signal currents to said recelving means, and means in said paths whereby said signal currents are conveyed through one or another of said paths 7 in accordance with the strength of said other currents.

20. In a signal receiving circuit, a plurality of conducting paths for the receiveds gnal currents, a receiver responsive to any substantial flow of signal current therethrough, and means to cause one said path to conduct said signal currents normally, and another of said paths to conduct said signal currents only when the first said path becomes overloads 21. In a signaling circuit, means for receiving small signal currents and large undesired currents, a plurality of conducting paths in parallel, and means to cause one said path to conduct said signal currents normally, and another of said paths to conduct said signal currents only when said large undesired currents are also present.

22. In a signaling circuit, means for receiving small signal currents and large undesired currents, a plurality of conducting paths in parallel, means to cause one said path to conduct said signal currents normally, and another of said paths to conduct said signal currents only when said large undesired currents are also present, and attenuating means in said other path.

23. .In a signal receiving circuit, a plurality of detectors to render signal currents perceptible, and means to cause one said detector to act on the signal currents normally and another said detector to acton said signal currents only when said first detector becomes overloaded.

24. In a signal receiving circuit, an amplifier and a plurality of detectors for acting on the signal currents, and means controlled by the received current for determining whether said signal currents are to be amplified by said amplifier or detected by one or another of said detectors.

25. In a signal receiving circuit, an amplifier upon which the signal currents are impressed, a receiver, and an attenuating means between said amplifier, and said receiver for variably attenuating the received currents in accordance with their magnitude.

- '26. In a signal receiving system, a terminal, a plurality of discharge devices having each a cathode, an anode and an impedance controlling electrode, said cathodes being connected together. and-having connection with said respective impedance controlling electrodes, means for electrically coupling said connection to said terminal, means for independently adjusting the potential of each said impedance controlling electrode, an anode branch connected to each anode and containing an adjustable impedance, a circ'uit connecting said cathodes in common to said anode branches in common, and a receiver. connected to said circuit.

27. The method of rendering signal current receivable in the presence of large disturbing current which comprises reducing the strength of both signal current and disturbing current in substantially the same proportion and impressing the ettect of both kinds of current upon the ear, the reduction of said currents being such as to bring the impressed energy Within the range of intensities in which the ear is able to discriminate between signal energy and disturbing energy.

28. An electrical network including a plurality of thermionic tubes arranged to transmit received current variations in parallel, means including a source of potential in the input circuit to each tube, of such value as to cause received current variations less than. a given magnitude to be transmitted by certain of said tubes and received current variations exceeding a given magnitude to be transmitted only by certain other of said tubes, and individual means associated With said tubes for attenuating the current variations transmitted by said tubes, the greater attenuation being associated with those tubes which transmit the larger curcircuits interposed between said incoming circuit and said translating device, and a current l miting means in each of said parallel circuits limiting the current in the successive parallel circuits to amplitudes of successively diiferent maxima.

31. In combination, an incoming circuit, a translating device, a plurality of parallel circuits interposed between said incoming circuit and said translating device, and an electron discharge current limiting device in each of said parallel circuits limiting the current in the successive parallel circuits to amplitudes of successively difl'erent maxima.

32. In asignaling system, an input circuit, a receiver, a plurality of parallel branch circuits interposed between said input circuit and receiver, a discharge device included in each of said parallel circuits, and means associated with said discharge devices to render said devices successively saturated when waves are impressed thereon.

In witness whereof, I hereunto subscribe my name this 30th day of April A. 1)., 1919.

CLARENCE A. SPRAGUE. 

